The invention relates to a composite casting process for making castings consisting in particular of light metal alloys reinforced by inserts, for example, of fiber-shaped or open-pored materials or the like, in particular, motor parts such as pistons, cylinders, cylinder heads and motor blocks of internal combustion engines, for example, in which process, firstly, a preform reinforced by the insert or inserts is made by embedding and/or the penetration of the insert(s) or an insert bundle, for example a fiber bundle, in molten matrix metal or by a molten matrix metal and subsequently solidifying it, then immersing it in a molten metal bath and subsequently inserting it into a casting mould for integrally casting or casting around the final casting.
Such a composite casting process is known from DE-PS-27 01 421 and DE-OS-35 11 542. This known processing method is particularly useful for the manufacture of larger and complicatedly structured fiber-reinforced castings and enables the required orientation of the fibers or whiskers to the main loading direction in the casting which is to be manufactured to be carried out in a manner relatively simple.
Therefore, the fiber or whisker reinforced preform must be made in a special casting process in which the matrix metal of the preform is forced into the fiber or whisker bundle at a controlled filling speed and at an exactly dosed pressure in order to ensure a faultless wetting of each individual fiber or whisker as well as the formation of a gap free substance-lacked bond and/or force-locked composite action between the fiber or whisker material and the matrix metal. The matrix metal is then allowed to solidify.
The subsequent integral casting or casting around of the final casting to or around the preform can then result by means of a simple casting process. The casting of the entire, final casting by means of the specialized casting process necessary for the manufacture of the preform will not be useful for the manufacture of larger and complicatedly structured castings as the required casting device would be too complicated and the casting parameters hardly controllable.
The initially described known composite casting process is, however, equally not without problems. Thus, the preform to be inserted into the casting mould is as a rule covered on its surface by an oxide skin which hinders or renders impossible a gapless metallurgical bond with the metal integrally cast or cast around. In order to have any chance at all of the formation of a metallurgical bond of the preform with the metal integrally cast or cast around, the preform must be inserted into the casting mould preheated to a relatively high temperature, which results in an increase in the oxide skin occuring on its surface. Thus, only an intensive flowing around of the preform with the integrally cast or cast around metal can lead to an oxide free bond.
In order to achieve such a faultless bond, in the known process according to DE-OS 35 11 542, the preform is submerged into a melt of a lead alloy heated to 150.degree. C.-400.degree. C. before its insertion into the casting mould in order to release its oxide skin. The lead alloy which adheres in this case is provided to prevent the renewed formation of an oxide layer on the metal surface of the preform prior to the integral casting or casting around of the final casting.
However, this known process has the disadvantage that the alloy elements of the lead melt enter into the bond layer between the precast and the integrally cast or cast around metal and can have an unforeseeable influence in this layer on the properties of the layer and under circumstances, even on the whole, final casting. Additionally, the preheating transmitted to the preform by a lead melt heated to only 150.degree.-400.degree. C. is as a rule not sufficient to ensure the complete bond of the preform with the integrally cast or cast around metal.
The melting regions of aluminium casting alloys lie between 540.degree. C. and 600.degree. C. A preform placed in the casting mould at a substantially lower temperature leads to the melt of the integrally cast or cast around solidifying immediately at the boundary surface to the preform so that the formation of a gapless metallurigal bond between this metal and the preform cannot be ensured in a sufficiently reliable manner.